Mail search doesn't find emails when searching by surname

Dears,

I had an hard time today to find an important email that I sent to a sender for which I recalled only the surname. Mail was unable to find it with just the surname. Then a friends of mine told me the exact username and then the search function of mail find it.

Here an example using myself, assume my is 'gborghi@gmail.com'. if I search for 'borghi' it do not find, if I search for 'gborghi' then it find the email. This is just an example to not share the real user name.

thanks for clarifying what's wrong.

kind regards

Giuseppe